Titanium dioxide, TiO2, is often prepared by starting with an aqueous solution of titanium tetrachloride, commonly referred to as titanium oxychloride, and adding a base, such as NaOH or NH4OH, to produce an amorphous or poorly crystalline hydrated titanium oxy-hydroxide precipitate, often called “titanyl hydroxide” or “titanium hydrolysate”, and also form a salt, such as NaCl or NH4Cl, that mostly dissolves in the aqueous solvent. This is illustrated in the following idealized reaction:TiOCl2(sol'n)+2NaOH(sol'n)→TiO(OH)2(ppt)+2NaCl(aq,s)
The titanium-containing precipitate can be readily isolated by gravity or vacuum filtration, and, optionally, the precipitate can be washed with water to remove residual metal or ammonium chloride reaction-product salt, and the precipitate can be calcined to convert it into crystalline TiO2.
The physical properties of the titanium-containing precipitate can vary depending upon the final slurry pH. The precipitate can be thick and composed of relatively large particle agglomerates when the final slurry pH is in the range 5-10. Solid from such a slurry is relatively facile to collect via gravity or vacuum filtration. As the slurry pH is lowered below about 5, the slurry becomes more fluid. Below pH˜3, the solids in the slurry become increasingly more difficult to filter and isolate for further processing. At a pH in the range of about 1-2 or lower, it has been found that the titanium-containing solid, comprising smaller particles, that settles on the filter membrane, compacts and transforms into a gelatinous material that becomes a barrier to liquid flow, resulting in a blocked, or “clogged”, filter.
A need exists for a process for making titanium dioxide particles, and, in particular, nano-sized titanium dioxide particles, that utilizes an acidic slurry that can be easily filtered before calcination to form the final product.